Devices and methods for electroluminescent display

ABSTRACT

In an active matrix electroluminescent display device, a storage capacitor ( 24 ) is provided for storing a voltage to be used for addressing a drive transistor ( 22 ). A discharge photodiode ( 27 ) is provided for discharging the storage capacitor in dependence on the light output of the display element, and an input data voltage applied to the pixel is changed by an amount corresponding to the threshold voltage of the drive transistor. The changed data voltage is applied between the gate and source of the drive transistor.In this device the initial voltage on the gate of the drive transistor is modified so as to remove the dependency of the light output on the threshold voltage, so that threshold voltage variations can be tolerated.

This invention relates to electroluminescent display devices,particularly active matrix display devices having thin film switchingtransistors associated with each pixel.

Matrix display devices employing electroluminescent, light-emitting,display elements are well known. The display elements may compriseorganic thin film electroluminescent elements, for example using polymermaterials, or else light emitting diodes (LEDs) using traditional III-Vsemiconductor compounds. Recent developments in organicelectroluminescent materials, particularly polymer materials, havedemonstrated their ability to be used practically for video displaydevices. These materials typically comprise one or more layers of asemiconducting conjugated polymer sandwiched between a pair ofelectrodes, one of which is transparent and the other of which is of amaterial suitable for injecting holes or electrons into the polymerlayer. The polymer material can be fabricated using a CVD process, orsimply by a spin coating technique using a solution of a solubleconjugated polymer. Ink-jet printing may also be used. Organicelectroluminescent materials exhibit diode-like I-V properties, so thatthey are capable of providing both a display function and a switchingfunction, and can therefore be used in passive type displays.Alternatively, these materials may be used for active matrix displaydevices, with each pixel comprising a display element and a switchingdevice for controlling the current through the display element.

Display devices of this type have current-addressed display elements, sothat a conventional, analogue drive scheme involves supplying acontrollable current to the display element. It is known to provide acurrent source transistor as part of the pixel configuration, with thegate voltage supplied to the current source transistor determining thecurrent through the display element. A storage capacitor holds the gatevoltage after the addressing phase.

FIG. 1 shows a known pixel circuit for an active matrix addressedelectroluminescent display device. The display device comprises a panelhaving a row and column matrix array of regularly-spaced pixels, denotedby the blocks 1 and comprising electroluminescent display elements 2together with associated switching means, located at the intersectionsbetween crossing sets of row (selection) and column (data) addressconductors 4 and 6. Only a few pixels are shown in the Figure forsimplicity. In practice there may be several hundred rows and columns ofpixels. The pixels 1 are addressed via the sets of row and columnaddress conductors by a peripheral drive circuit comprising a row,scanning, driver circuit 8 and a column, data, driver circuit 9connected to the ends of the respective sets of conductors.

The electroluminescent display element 2 comprises an organic lightemitting diode, represented here as a diode element (LED) and comprisinga pair of electrodes between which one or more active layers of organicelectroluminescent material is sandwiched. The display elements of thearray are carried together with the associated active matrix circuitryon one side of an insulating support. Either the cathodes or the anodesof the display elements are formed of transparent conductive material.The support is of transparent material such as glass and the electrodesof the display elements 2 closest to the substrate may consist of atransparent conductive material such as ITO so that light generated bythe electroluminescent layer is transmitted through these electrodes andthe support so as to be visible to a viewer at the other side of thesupport. Typically, the thickness of the organic electroluminescentmaterial layer is between 100nm and 200nm. Typical examples of suitableorganic electroluminescent materials which can be used for the elements2 are known and described in EP-A-0 717446. Conjugated polymer materialsas described in WO96/36959can also be used.

FIG. 2 shows in simplified schematic form a known pixel and drivecircuitry arrangement for providing voltage-addressed operation. Eachpixel 1 comprises the EL display element 2 and associated drivercircuitry. The driver circuitry has an address transistor 16 which isturned on by a row address pulse on the row conductor 4. When theaddress transistor 16 is turned on, a voltage on the column conductor 6can pass to the remainder of the pixel. In particular, the addresstransistor 16 supplies the column conductor voltage to a current source20, which comprises a drive transistor 22 and a storage capacitor 24.The column voltage is provided to the gate of the drive transistor 22,and the gate is held at this voltage by the storage capacitor 24 evenafter the row address pulse has ended.

The drive transistor 22 in this circuit is implemented as a PMOS TFT, sothat the storage capacitor 24 holds the gate-source voltage fixed. Thisresults in a fixed source-drain current through the transistor, whichtherefore provides the desired current source operation of the pixel.

In the above basic pixel circuit, different transistor characteristicsacross the substrate (particularly the threshold voltage) give rise todifferent relationships between the gate voltage and the source-draincurrent, and artefacts in the displayed image result. In addition tothese threshold voltage variations, differential aging of the LEDmaterial gives rise to variations in image quality across a display.

It has been recognised that a current-addressed pixel (rather than avoltage-addressed pixel) can reduce or eliminate the effect oftransistor variations across the substrate. For example, acurrent-addressed pixel can use a current mirror to sample thegate-source voltage on a sampling transistor through which the desiredpixel drive current is driven. The sampled gate-source voltage is usedto address the drive transistor. This partly mitigates the problem ofuniformity of devices, as the sampling transistor and drive transistorare adjacent each other over the substrate and can be more accuratelymatched to each other. Another current sampling circuit uses the sametransistor for the sampling and driving, so that no transistor matchingis required, although additional transistors and address lines arerequired.

There have also been proposals for voltage-addressed pixel circuitswhich compensate for the aging of the LED material. For example, variouspixel circuits have been proposed in which the pixels include a lightsensing element. This element is responsive to the light output of thedisplay element and acts to leak stored charge on the storage capacitorin response to the light output, so as to control the integrated lightoutput of the display during the address period. FIG. 3 shows oneexample of pixel layout for this purpose. Examples of this type of pixelconfiguration are described in detail in WO 01/20591and EP 1 096 466.

In the pixel circuit of FIG. 3, a photodiode 27 discharges the gatevoltage stored on the capacitor 24. The EL display element 2 will nolonger emit when the gate voltage on the drive transistor 22 reaches thethreshold voltage, and the storage capacitor 24 will then stopdischarging. The rate at which charge is leaked from the photodiode 27is a function of the display element output, so that the photodiode 27functions as a light-sensitive feedback device. It can be shown that theintegrated light output, taking into the account the effect of thephotodiode 27, is given by:

$\begin{matrix}{L_{T} = {\frac{C_{S}}{\eta_{PD}}\left( {{V(0)} - V_{T}} \right)}} & \lbrack 1\rbrack\end{matrix}$

In this equation, η_(PD)is the efficiency of the photodiode, which isvery uniform across the display, C_(S)is the storage capacitance, V(0)is the initial gate-source voltage of the drive transistor and V_(T)isthe threshold voltage of the drive transistor. The light output istherefore independent of the EL display element efficiency and therebyprovides aging compensation. However, V_(T)varies across the display soit will exhibit non-uniformity. Reference is made to the paper “Acomparison of pixel circuits for Active Matrix Polymer/Organic LEDDisplays” by D. A. Fish et al., 32.1, SID 02Digest, May 2002.

There are refinements to this basic circuit, but the problem remainsthat practical voltage-addressed circuits are still susceptible tothreshold voltage variations.

According to a first aspect of the invention, there is provided anactive matrix electroluminescent display device comprising an array ofdisplay pixels, each pixel comprising:

-   -   an electroluminescent display element;    -   a drive transistor for driving a current through the display        element;    -   a storage capacitor for storing a voltage to be used for        addressing the drive transistor;    -   a discharge photodiode for discharging the storage capacitor in        dependence on the light output of the display element; and    -   circuit elements for changing an input data voltage applied to        the pixel by an amount corresponding to the threshold voltage of        the drive transistor, and for applying the changed data voltage        between the gate and source of the drive transistor.

In this pixel arrangement, circuitry is provided for modifying theinitial voltage on the gate of the drive transistor. With reference toequation [1] above, this has the effect of removing the dependency ofthe light output on the threshold voltage, so that threshold voltagevariations can be tolerated.

As in the conventional circuits, each pixel comprises an addresstransistor connected between a data signal line and an input to thepixel, and the drive transistor is connected between a power supply lineand the display element.

In a first embodiment, the storage capacitor is connected between thepower supply line and the gate of the drive transistor. Thus, thestorage capacitor stores the gate-source voltage of the drivetransistor. In order to modify the pixel drive voltage, the circuitelements in this embodiment comprise a second photodiode and a secondstorage capacitor, wherein the second photodiode is connected betweenthe gate of the drive transistor and one terminal of the second storagecapacitor, and the discharge photodiode is connected between the oneterminal and the power supply line.

In this arrangement, a second storage capacitor is used for chargepumping. At the end of a frame, the voltage on the gate of the drivetransistor is the threshold voltage, because this is the voltage atwhich the transistor turns off. The circuit of this embodiment acts toadd a drive voltage to the threshold voltage already stored on the firststorage capacitor, through capacitive coupling, namely charge pumping.By ensuring the voltage on the storage capacitor is increased by a drivevoltage, rather than charged to the drive voltage, the dependency on thethreshold voltage is removed.

In this arrangement, the data input to the pixel is supplied to thesecond terminal of the second storage capacitor.

The LED should be turned off during the addressing phase, so that thephotodiodes have minimum influence on the charge pumping operation. Forthis purpose, an isolating transistor is preferably connected betweenthe drive transistor and the display element.

In a second embodiment, the storage capacitor is again connected betweenthe power supply line and the gate of the drive transistor, and thephotodiode is connected between the power supply line and the gate ofthe drive transistor. The circuit elements comprise two paralleloppositely facing diode-connected transistors, connected between theinput to the pixel and the gate of the drive transistor. In thisarrangement, a diode-connected transistor provides a voltage drop whichequates to the threshold voltage (if the diode-connected transistor ismatched to the drive transistor) between the voltage input to the pixeland the voltage stored on the storage capacitor. The voltage drop acrossthe diode-connected transistor translates to an increased voltage acrossthe storage capacitor (because it is connected to the power supply line)thereby removing the dependency of the light output on the thresholdvoltage.

In a third embodiment, the storage capacitor and the dischargephotodiode are connected in parallel between the power supply line andan input to the pixel, and the circuit elements comprise a thresholdstorage capacitor connected between the input and the gate of the drivetransistor.

In this arrangement, the storage capacitor does not store the desiredsource-gate voltage of the drive transistor. Instead, the storagecapacitor stores the input drive voltage, and a series-connectedthreshold storage capacitor provides a voltage shift between the storagecapacitor and the gate of the drive transistor. Additional circuitry isrequired to enable the threshold voltage to be stored on the thresholdstorage capacitor. For example, the circuit elements may furthercomprise a bypass transistor connected between the source and gate ofthe drive transistor for charging the threshold storage capacitor to thethreshold voltage using a current of the drive transistor.

According to a second aspect of the invention, there is provided anactive matrix electroluminescent display device comprising an array ofdisplay pixels, each pixel comprising:

-   -   an electroluminescent display element;    -   a current sampling circuit for sampling a drive current and        including a drive transistor for driving current through the        display element;    -   a storage capacitor for storing a gate-source voltage for the        drive transistor corresponding to the sampled drive current; and    -   a photodiode for discharging the storage capacitor in dependence        on the light output of the display element.

In this arrangement, a current sampling circuit is used to sample adrive current. This enables threshold voltage variations to be avoided.The photodiode additionally enables aging compensation to beimplemented.

In one embodiment of the second aspect of the invention, the currentsampling circuit comprises an isolating transistor for selectivelyisolating the drive transistor from the display element and a bypasstransistor for selectively connecting the drive transistor to the inputof the pixel. This current sampling circuit uses the drive transistorfor the current sampling. Other circuits are also possible which act ascurrent mirrors, with separate current sampling and current drivetransistors.

The first aspect of the invention also provides a method of driving anactive matrix electroluminescent display device comprising an array ofdisplay pixels each comprising a drive transistor and anelectroluminescent display element, the method comprising, for eachaddressing of the pixel:

-   -   applying a drive voltage to an input of the pixel;    -   modifying the drive voltage by an amount corresponding to the        threshold voltage of the drive transistor;    -   storing the modified drive voltage in a capacitor arrangement        and applying the modified drive voltage to the gate of the drive        transistor, thereby compensating for threshold variations        between drive transistors of different pixels; and    -   discharging the capacitor arrangement using a photodiode        illuminated by the light output of the electroluminescent        display element, thereby compensating for aging variations        between pixels.

This method provides the optical feedback discharge of the storagecapacitor for aging compensation, in combination with threshold voltagecompensation.

Storing the modified drive voltage can comprise:

-   -   storing the modified drive voltage on a capacitor;    -   storing the drive voltage on a first capacitor and storing a        voltage corresponding to the threshold voltage of the drive        transistor on a second capacitor; or    -   pumping the drive voltage onto a storage capacitor on which a        voltage corresponding to the threshold voltage was previously        provided.

The second aspect of the invention also provides a method of driving anactive matrix electroluminescent display device comprising an array ofdisplay pixels each comprising a drive transistor and anelectroluminescent display element, the method comprising, for eachaddressing of the pixel:

-   -   applying a drive current to an input of the pixel;    -   sampling the drive current to obtain a gate-source voltage of        the drive transistor corresponding to the drive current;    -   storing the gate-source voltage on a storage capacitor;    -   applying the gate-source voltage to the drive transistor; and    -   discharging the storage capacitor using a photodiode illuminated        by the light output of the electroluminescent display element.

This method uses current addressing to provide threshold compensationbut additionally uses the optical feedback discharge of the storagecapacitor for aging compensation.

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 shows a known EL display device;

FIG. 2 is a simplified schematic diagram of a known pixel circuit forcurrent-addressing the EL display pixel;

FIG. 3 shows a known pixel design which compensates for differentialaging;

FIG. 4 shows a first example of pixel circuit according to theinvention;

FIG. 5 shows a second example of pixel circuit according to theinvention;

FIG. 6 shows a third example of pixel circuit according to theinvention; and

FIG. 7 shows a fourth example of pixel circuit according to theinvention.

It should be noted that these figures are diagrammatic and not drawn toscale. Relative dimensions and proportions of parts of these figureshave been shown exaggerated or reduced in size, for the sake of clarityand convenience in the drawings.

In accordance with the invention, the pixel circuitry is modified sothat an input data voltage applied to the pixel can be changed by anamount corresponding to the threshold voltage of the drive transistor.This is in addition to the use of a photodiode to removing agingfluctuations. This enables the initial voltage on the gate of the drivetransistor to be modified, so that in equation [1] above, this has theeffect of removing the dependency of the light output on the thresholdvoltage, so that threshold voltage variations can be tolerated.

FIG. 4 shows a first example of pixel layout of the invention. The samereference numerals are used to denote the same components as in FIGS. 2and 3, and the pixel circuit is for use in a display such as shown inFIG. 1.

The storage capacitor 24 is again connected between the power supplyline 26 and the gate of the drive transistor 22. Thus, the storagecapacitor stores the gate-source voltage of the drive transistor 22. Inorder to modify the pixel drive voltage, a second photodiode 30 and asecond storage capacitor 32 are provided. The second photodiode 30 isconnected between the gate of the drive transistor 22 and one terminalof the second storage capacitor 32, and the discharge photodiode 27 isconnected between that one terminal and the power supply line 26. Theinput to the pixel is supplied by the address transistor 16 to the otherterminal of the second storage capacitor 32.

As will be apparent from the following, the second storage capacitor 32is used for charge pumping. In particular, at the end of a frame period,the voltage on the gate of the drive transistor 22 is the thresholdvoltage, because this is the voltage at which the drive transistor 22turns off. Furthermore, the second storage capacitor 32 is uncharged ascharge is removed from it at the end of the address phase. The drivevoltage is added by charge pumping to the threshold voltage alreadystored on the first storage capacitor 24.

At the beginning of an addressing phase, the NMOS address transistor 16is turned on by a high pulse on the row conductor 4. A second transistor34 (functioning as an isolating device) is provided between the drivetransistor 22 and the display element 2, and this is a PMOS device.Thus, the high addressing pulse on the row conductor 4 turns on theaddress transistor 16 and simultaneously turns off the transistor 34 sothat the EL display element 2 is switched off during the addressingphase.

The pixel drive voltage on the column conductor 6 is low with respect tothe power supply line voltage 26, so that when the drive voltage isapplied, the second photodiode 30 is forward biased and current flowsthrough it, sourced from the capacitor 24 which had a voltage drop ofonly the drive transistor threshold voltage. This current charges thesecond capacitor 32 until an equilibrium is reached, and at this point,the voltage across the storage capacitor 24 has a value dependent on theinitial threshold voltage and on the pixel drive voltage applied to thecolumn 6, and additionally dependent on the ratios of the capacitancesof 24 and 32.

If the capacitance of the storage capacitor 24 is much greater than thecapacitance of the second capacitor 32 (C₂₄>>C₃₂), then the finalvoltage across the storage capacitance is approximately equal to thethreshold voltage V_(T) plus a factor (C₃₂/C₂₄) of the drive voltage.This requires large voltage swings for the drive voltage, as the drivevoltage is reduced by the C₃₂/C₂₄ factor.

During the addressing phase, the second transistor 34 is turned off, sothat there is no illumination of the photodiodes 27, 30 and nosignificant additional minority carrier currents flow in thephotodiodes. The photodiodes are screened from external illumination.

At the end of the addressing phase, the column 6 is driven to a highvoltage so that the photodiode 27 is forward biased and the charge onthe second capacitor 32 is removed, but the charge on the first storagecapacitor 24 is left unchanged. At the end of the addressing phase, theaddressing transistor 16 is turned off and the second transistor 34 isturned on, and the pair of photodiodes 27, 30 act to decay the charge onthe storage capacitor 24 until the threshold voltage is reached and thedrive transistor 22 is turned off.

The initial voltage on the storage capacitor at the end of theaddressing phase is now:V(0)=f ₁(V _(data))+f ₂(V _(T))

Where f1 and f2 are functions dependent on the relative capacitances ofcapacitors 24 and 32 and V_(data) is the voltage applied to the columnconductor 6. As mentioned above, f₂ can be made to approximate to 1bysuitable selection of the capacitances. By ensuring the voltage on thestorage capacitor is increased in dependence on the drive voltage,rather than charged to the drive voltage, the dependency on thethreshold voltage can be removed. In particular, the integrated lightoutput of equation [1] becomes:

$\begin{matrix}{L_{T} = {\frac{C_{S}}{\eta_{PD}}{f\left( V_{DATA} \right)}}} & \lbrack 2\rbrack\end{matrix}$

As mentioned above, this embodiment requires large voltage swings inV_(data), and further embodiments below avoid this requirement.

FIG. 5 shows a second embodiment, in which the storage capacitor 24 andthe discharge photodiode 27 are connected in parallel between the powersupply line 26 and an input to the pixel (namely the output of theaddress transistor 16).

The circuit has a threshold storage capacitor 40 connected between theinput and the gate of the drive transistor 22. In this arrangement, thestorage capacitor 24 does not store the desired source-gate voltage ofthe drive transistor 22. Instead, the storage capacitor 24 stores theinput drive voltage, and the series-connected threshold storagecapacitor 40 provides a voltage shift between the storage capacitor andthe gate of the drive transistor 22.

In order to provide the threshold voltage across threshold storagecapacitor 40, a bypass transistor 42 is connected between the source andgate of the drive transistor for charging the threshold storagecapacitor 40 to the threshold voltage using a current of the drivetransistor. As in the example of FIG. 4, an additional isolatingtransistor 34 is provided between the drive transistor 22 and thedisplay element 2, and provided with its own address line 35.

During the addressing phase for this circuit, the addressing transistor16 is initially turned on to store a constant initial voltage on thestorage capacitor 24. This constant voltage is the power supply linevoltage so that capacitor 24 is discharged and the photodiode 27 isshorted. The address transistor 16 can then be turned off. The isolatingtransistor 34 is turned on (or it may have been on since the beginningof the address phase), so that a current is driven through the ELdisplay element. An ON-current thus passes through the drive transistor22. The bypass transistor 42 is then turned on, and the isolatingtransistor is turned off. The drive transistor 22 remains on, as thegate-source voltage has not changed, but the drive current of the drivetransistor 22 passes through the bypass transistor 42 to the thresholdstorage capacitor 40.

When sufficient charge has passed to the threshold storage capacitor 40,the voltage on the terminal connected to the drive transistor gatereaches a level when the PMOS drive transistor turns off. At this point,the threshold voltage of the drive transistor 22 is stored on thethreshold storage capacitor 40.

The bypass transistor 42 is then turned off and the storage capacitor 24is charged to the desired data voltage, by applying the data voltage tothe column 6 and switching on the address transistor 16.

The photodiode action thus only takes place when the second transistor34 is turned on at the end of the address sequence, and the thresholdstorage capacitor 40 introduces a step voltage change between thevoltage on the storage capacitor 24 and the voltage applied to the gateof the drive transistor 24. Again, by ensuring the voltage applied tothe gate is increased relative to the source (namely decreased inabsolute terms) by the threshold voltage, the dependency on thethreshold voltage is removed.

FIG. 6 shows a third embodiment in which the storage capacitor 24 andphotodiode 27 are again connected between the power supply line 26 andthe gate of the drive transistor 22. Two parallel oppositely facingdiode-connected transistors 50, 52 are connected between the input tothe pixel (the output of the address transistor 16) and the gate of thedrive transistor 22. One of the diode-connected transistors provides avoltage drop of the threshold voltage and to provide this thediode-connected transistor is matched to the drive transistor 22. Thisvoltage drop between the voltage input to the pixel and the voltagestored on the storage capacitor 24 results in an increase of thegate-source voltage on the drive transistor 22 by the same amount. Thisagain removes the dependency of the light output on the thresholdvoltage.

The second diode-connected transistor is required for the resetting ofthe pixel.

The above pixel designs show some possible implementations ofvoltage-addressed pixels having aging compensation implemented usingphotodiode optical feedback circuits and with threshold compensationimplemented in various ways.

The invention can also provide current-addressed implementations. FIG. 7shows an arrangement in which a current sampling circuit is used tosample a drive current. This enables threshold voltage variations to beavoided. The photodiode additionally enables aging compensation to beimplemented.

In FIG. 7, the current sampling circuit comprises the additionaltransistor 34 for selectively isolating the drive transistor 22 from thedisplay element 2 and a bypass transistor 60 for selectively connectingthe drive transistor 22 to the input of the pixel (again this input istaken to be the output of the address transistor 16).

To sample an input current, the bypass transistor 60 is turned on andthe additional transistor 34 is turned off. The input current is thusdriven through the drive transistor 22. The storage capacitor is chargedto the corresponding gate-source voltage of the drive transistor 22, andsubsequently drives the drive transistor 22. This current samplingcircuit uses the drive transistor for the current sampling, and thesampling operation takes into account the transistor characteristics, sothat threshold variations are avoided.

Other circuits are also possible which act as current mirrors, withseparate current sampling and current drive transistors—these do,however, require matched transistor characteristics.

The voltage addressed circuits described above all operate by modifyingthe drive voltage by an amount corresponding to the threshold voltage ofthe drive transistor. This modified drive voltage is stored in one ormore capacitors and applied to the gate of the drive transistor, therebycompensating for threshold variations between drive transistors ofdifferent pixels. In addition, capacitor discharge using a photodiodeilluminated by the light output of the electroluminescent displayelement compensates for aging variations between pixels. The circuitsabove are only examples of possible circuits for this purpose, and otherimplementations will be apparent to those skilled in the art.

The current addressed circuit described above samples an input drivecurrent to obtain a gate-source voltage of the drive transistorcorresponding to the drive current. This gate-source voltage is storedand applied to the drive transistor. Again, capacitor discharge using aphotodiode illuminated by the light output of the electroluminescentdisplay element compensates for aging variations between pixels. Thecircuit above is only one example of a possible current-addressedimplementation and other implementations will be apparent to thoseskilled in the art.

The specific examples above also use different combinations of NMOS andPMOS transistors, and it will be understood that other specificimplementations will be apparent.

1. An active matrix electroluminescent display device comprising anarray of display pixels, each pixel comprising: an electroluminescentdisplay element (2); a drive transistor (22) connected between a powersupply line (26) and the display element (2) for driving a currentthrough the display element (2); a storage capacitor (24) for storing avoltage to be used for addressing the drive transistor; a dischargephotodiode (27) for discharging the storage capacitor (24) in dependenceon the light output of the display element; and circuit elements forchanging an input data voltage applied to the pixel by an amountcorresponding to the threshold voltage of the drive transistor, and forapplying the changed data voltage between the gate and the source of thedrive transistor (22), wherein the circuit elements comprise a secondphotodiode (30) and a second storage capacitor (32),.wherein the secondphotodiode (30) is connected between the gate of the drive transistor(22) and one terminal of the second storage capacitor (32), and thedischarge photodiode (27) is connected between the one terminal and thepower supply line (26).
 2. A device as claimed in claim 1, wherein eachpixel further comprises an address transistor (16) connected between adata signal line (6) and an input to the pixel.
 3. A device as claimedin claim 1, wherein the storage capacitor (24) is connected between thepower supply line (26) and the gate of the drive transistor.
 4. A deviceas claimed in claim 1,wherein data input to the pixel is supplied to theother second terminal of the second storage capacitor (32).
 5. A deviceas claimed in claim 1,wherein the circuit elements further comprise anisolating transistor (34) connected between the drive transistor (22)and the display element (2).
 6. A device as claimed in claim 3, whereinthe photodiode (27) is connected between the power supply line (26) andthe gate of the drive transistor (22), and the circuit elements comprisetwo parallel oppositely facing diode-connected transistors (50, 52)connected between the input to the pixel and the gate of the drivetransistor (22).
 7. A device as claimed in claim 1,wherein the storagecapacitor (24) and the discharge photodiode (27) are connected inparallel between the power supply line (26) and an input to the pixel,and the circuit elements comprise a threshold storage capacitor (40)connected between the input and the gate of the drive transistor.
 8. Adevice as claimed in claim 7, wherein the circuit elements furthercomprise a bypass transistor (42) connected between the source and gateof the drive transistor (22) for charging the threshold storagecapacitor (40) to the threshold voltage using a current of the drivetransistor (22).
 9. An active matrix electroluminescent display devicecomprising an array of display pixels, each pixel comprising: anelectroluminescent display element (2); a current sampling circuit forsampling a drive current and including a drive transistor (22) fordriving current through the display element; a storage capacitor (24)for storing a gate-source voltage for the drive transistor (22)corresponding to the sampled drive current; a photodiode (27) fordischarging the storage capacitor (24) in dependence on the light outputof the display element and circuit elements for changing an input datavoltage applied to the pixel by an amount corresponding to the thresholdvoltage of the drive transistor, and for applying the changed datavoltage between the gate and the source of the drive transistor (22),wherein the circuit elements comprise a second photodiode (30) and asecond storage capacitor (32), wherein the second photodiode (30) isconnected between the gate of the drive transistor (22) and one terminalof the second storage capacitor (32). and the discharge photodiode (27)is connected between the one terminal and the power supply line (26).10. A device as claimed in claim 9, wherein the current sampling circuitcomprises an isolating transistor (34) for selectively isolating thedrive transistor (22) from the display element (2) and a bypasstransistor (60) for selectively connecting the drive transistor (22) tothe input of the pixel.
 11. A method of driving an active matrixelectroluminescent display device comprising an array of display pixelseach comprising a drive transistor (22) and an electroluminescentdisplay element (2), the method comprising, for each addressing of thepixel: applying a drive voltage to an input of the pixel; modifying thedrive voltage by an amount corresponding to the threshold voltage of thedrive transistor (22); storing the modified drive voltage in a capacitorarrangement and storing the modified drive voltage to the gate of thedrive transistor, thereby compensating for threshold variations betweendrive transistors of different pixels; and discharge the capacitorarragement using a photodiode(27)illuminated by the light output of theelectroluminescent display element, thereby compensating for agingvariations between pixels, wherein strong the modified drive voltagecomprises strong the drive voltage on a first capacitor (24) and storinga voltage cerresponding to the threshold voltage of the drive transistoron a seccond capacitor (40).
 12. A method as claimed in claim 11,wherein storing the modified drive voltage further comprises pumping thedrive voltage onto a storage capacitor (24) on which a voltagecorresponding to the threshold voltage was previously provided.
 13. Amethod of driving an active matrix electroluminescent display devicecomprising an array of display pixels each comprising a drive transistor(22) and an electroluminescent display element (2), the methodcomprising, for each addressing of the pixel: applying a drive currentto an input of the pixel; sampling the drive current to obtain agate-source voltage of the drive transistor corresponding to the drivecurrent; storing the gate-source voltage on a storage capacitor (24);applying the gate-source voltage to the drive transistor; anddischarging the storage capacitor using a photodiode illuminated by thelight output of the electroluminescent display element, wherein storingthe gate-source voltage comprises storing the gate-source voltage on afirst capacitor (24) and storing a voltage corresponding to thethreshold voltage of the drive transistor on a second capacitor (40).